The present invention relates to carrier sense multiple access with collision detect communication methods, and in particular to the optimization of frame usage therein.
In current carrier sense multiple access with collision detect (CSMA/CD) communication protocols, several devices are coupled by a communications cable/bus in a network. If a first device has data to send to another device, it senses the carrier or cable to see if another device is presently transmitting data. If no other device is using the cable, the first device transmits the data and also listens on the line to make sure that the data did not collide with data being sent by another device. The data is packaged into an entity called a frame consistent with IEEE standard 802.3, CSMA/CD Access Method and Physical Layer Specifications. The frame, in addition to the actual data, includes information to identify the source and destination of the frame, type of frame, start of frame, checking data, preamble and a pad area.
Frame length is agreed upon by the devices, and should be long enough so that collisions can always be detected. It has been varied based on the amount of traffic on a network in an effort to maximize bandwidth utilization. U.S. Pat. No. 4,543,654 to Jones describes a way of ensuring that the preamble is long enough to ensure collision detection. In this patent, the size of the preamble is varied depending on the propagation characteristics of the communication network.
One problem associated with CSMA/CD networks is that only one record can be transmitted at a time. This can cause difficulties if the size of a record is larger or smaller than the room left in a frame for data. One can try to optimize the size of the frames to accommodate the average size of data, but the frame must still be large enough to ensure collision detection. U.S. Pat. No. 4,771,391 describes a method of optimizing the length of information packets to minimize delay in a node gaining access to a network. In a different communication protocol as described in U.S. Pat. No. 4,688,035 to Gray et al., a "more bit" in a length of data field was used to indicate that a further part of a record will be coming in a future frame. In the IEEE standard, there is no field defined for insertion of a "more bit."
Where the data comprises several smaller records, they are sent one at a time with the pad field increased to fill up the rest of the frame. This is quite wasteful from at least two perspectives. First, when each frame is sent, there is the possibility of a collision. If a collision occurs, the frame must be present at a later time as determined by a retry protocol. Since the data is broken into multiple frames, each of which is subject to a new chance of collision, there is an increased chance of collision for transmission of the record. Further, if all of the data from multiple devices are being broken apart to be sent, there are many more frames being transmitted, again increasing the chance of collision.